1. Field of the Invention
The present invention relates to a system for adjusting the idle speed of an internal combustion engine through feedback control.
2. Description of the Related Art
In general, the flow rate of intake air taken into an internal combustion engine is closely related to the rate of supply of a fuel to the engine. It is known that the speed of running of the engine can be varied by varying the rate at which air is taken into the engine.
A known idle speed adjusting system for an internal combustion engine will be described with specific reference to FIG. 1. Referring to this figure, an internal combustion engine 1 has an intake pipe 2 which is provided with a throttle valve 3. A bypass intake passage 9 directly connects portions of the intake passage 2 upstream and downstream of the throttle valve 3 so as to bypass the throttle valve 3. More specifically, the bypass intake passage 9 includes a main bypass passage 91 and an auxiliary bypass passage 92 which are parallel to each other. The main bypass passage 91 has a linear solenoid valve 8 (referred to simply as "solenoid valve" hereinafter) capable of varying the cross-sectional area of the air passage in the main bypass passage 91 in accordance with the electrical current supplied thereto. Thus, the solenoid valve 8 functions as an intake air control valve. An adjusting screw 4 provided in the auxiliary bypass passage 92 is capable of varying the cross-sectional area of the air passage so as to control the flow rate of air flowing through the auxiliary bypass passage 92. The solenoid valve 8 is arranged so as to be driven and controlled by the output of an actuator 7.
On the other hand, a gear 41 provided on the shaft of the internal combustion engine 1 rotates as the engine operates, and the rotation of this gear 41 is sensed by a rotation speed sensor 42. The rotation speed sensor 42 detects the rotation speed of the gear 41 and delivers the engine speed n.sub.E to an offset amplifier 61. The offset amplifier 61 also receives a command speed n.sub.T from a command speed generator 5. The offset amplifier 61 computes the offset .DELTA.n of the engine speed n.sub.E from the command speed n.sub.T and delivers it to a speed controller 62. The command speed generator 5 generates, in accordance with conditions such as the engine temperature, a predetermined idle speed as the command speed n.sub.T. The speed controller 62, upon receipt of the speed offset .DELTA.n, conducts a proportional, integrating or differentiation operation so as to generate a speed correction signal S.sub.C which acts in the direction to cancel the speed offset .DELTA.n. Meanwhile, a reference control amount output circuit 11 delivers a reference control signal S.sub.T representing a fixed control amount to maintain the engine speed n.sub.E at the same level as the command speed n.sub.T. An adder 13 adds this reference control signal S.sub.T to the output S.sub.C from the speed controller 62, and delivers the sum as an output. The output (S.sub.T +S.sub.C) of the adder 13 is delivered to a limiter 12, which delivers a signal representing a limited value within the range of (S.sub.T +S.sub.C). The output from the limiter 12 is delivered to the actuator 7 which delivers to the solenoid valve 8, upon receipt of the output from the limiter 12, an actuating signal of a duty cycle corresponding to the input signal. The solenoid valve 8 then increases or decreases the cross-sectional area of the air passage, thereby controlling the flow rate of air flowing through the bypass intake passage 9.
A description will now be given of the operation of this system. The speed controller 62 operates in accordance with the speed offset .DELTA.n, thereby generating a speed correction signal S.sub.C. This speed correction signal S.sub.C varies in such a direction as to reduce the speed offset .DELTA.n which is output from the offset amplifier 61, and is settled when the speed offset .DELTA.n is minimized. The adder 13 adds the output S.sub.C of the speed controller 62 to the output S.sub.T from the reference control output circuit 11 and delivers the sum to the limiter 12. The limiter 12 delivers a limited output to the actuator 7 which in turn produces an actuating signal for actuating the solenoid valve 8.
The idle speed is adjusted in a manner which will be explained hereinunder. It is assumed here that the throttle valve 3 is in the idle position and the control is conducted when the engine is sufficiently warmed up. A correction value output circuit 20 converts the speed correction signal S.sub.C from the speed controller 62 into a duty cycle signal as shown in FIG. 2, and delivers this signal to an externally connected meter 21. The meter 21 is a volt meter capable of indicating a value corresponding to the mean voltage. The adjusting screw 4 on the bypass intake passage 9 is then manually adjusted such that the meter indicates a value corresponding to a 50% duty ratio. Consequently, the speed correction signal S.sub.C is reduced to zero, whereby the offset of the idle speed is eliminated regardless of the causes of the offset, e.g., clogging of the throttle valve 3 or of the solenoid valve 8, and so forth.
A description will now be given of another known idle speed adjusting system for an internal combustion engine, with specific reference to FIG. 3. In FIG. 3, the same reference numerals are used to denote the same components as those appearing in FIG. 1, and detailed description of such components is omitted. The engine speed n.sub.E sensed by the speed sensor 42 is delivered not only to the offset amplifier 61 but also to an engine speed meter 60. The limiter 12 delivers a signal of a value within a limited range in response to the input value (S.sub.T +S.sub.C), in accordance with the operations of the command speed generator 5, the offset amplifier 61, the speed controller 62, the reference control amount output circuit 11 and the adder 13. A switching circuit 15 selects the output of the limiter 12 when the idle speed adjusting switch 14 is off, whereas, when the idle speed adjusting switch 14 is on, it selects the output S.sub.T from the reference control amount output circuit 11. The output selected by the switching circuit 15, i.e., the output from the limiter 12, or the output from the reference control amount output circuit 11, is delivered to the actuator 7 which, in accordance with the selected output, delivers to the solenoid valve 8 a duty cycle signal corresponding to the selected output. In consquence, the solenoid valve 8 controls the cross-sectional area of the air passage so as to increase and decrease the flow rate of the intake air flowing through the bypass intake passage 9. The idle speed adjusting switch 14 applies a predetermined signal to the input terminal of the switching circuit 15.
A description will now be given of the operation of this system. The speed controller 62 operates in accordance with the speed offset .DELTA.n, so as to produce a speed correction signal S.sub.C. The speed correction signal S.sub.C is generated in such a direction as to reduce the offset .DELTA.n which is output from the offset amplifier 61, and is settled when the offset .DELTA.n is minimized. The output S.sub.C of the speed controller 62 is added by the adder 13 to the output S.sub.T of the reference control amount output circuit 11 and the sum is delivered to the limiter 12. Either the output of the reference control amount output circuit 11 or the output of the limiter 12 is delivered to the actuator 7 in response to the ON or OFF of the idle adjusting switch 14, and is converted to an actuating signal for actuating the solenoid valve 8.
The manner in which the idle speed is adjusted in this system will be described with reference to FIG. 3. The adjustment of the idle speed is conducted, for example, when the engine has been sufficiently warmed up, with the throttle valve 3 being set at the idle position. A mechanic then turns on the idle adjusting switch 14. As a consequence, the reference control signal S.sub.T from the reference control amount output circuit 11 is selected by the switching circuit 15 and is then delivered to the actuator 7. The actuator 7 delivers an actuating signal of a duty cycle corresponding to the level of the reference control signal S.sub.T to the solenoid valve 8 so as to set the solenoid valve 8 to a reference degree of opening. On the other hand, the engine speed meter 60 receiving a signal output which indicates the engine speed n.sub.E from an engine speed sensor 42. In this state, the mechanic manually adjusts the adjusting screw 4 to adjust the intake airflow rate through the auxiliary bypass intake passage, while monitoring the engine speed meter 60, thereby setting the engine speed n.sub.E to a predetermined speed.
In the known idle speed adjusting systems having the described constructions, the intake air flow rate varies according to the value of the electrical current flowing through the linear solenoid of the solenoid valve 8. The electrical current through the linear solenoid varies according to the electrical resistance of the linear solenoid which varies according to the ambient air temperature. This poses the following problem. If the idle speed is adjusted when the ambient air temperature is high, since the linear solenoid of the solenoid valve 8 exhibits a large resistance, the intake air flow rate is decreases, requiring the adjusting screw to be opened more than when the adjustment is conducted at a lower ambient air temperature. Therefore, when the air temperature drops, the electrical resistance of the linear solenoid of the solenoid valve 8 is changed to cause a change in the intake air flow rate. In such a case, though a demand for closing the intake air control valve exists for the purpose of maintaining the idle speed at the command level, the control of the intake air flow rate may be defective after the solenoid valve 8 is at its lower limit of control. Consequently, the idling speed is set at a higher rate than the command idle speed, resulting in an uneconomical use of fuel.